Inhibitors of histone deacetylases (HDAC) have been shown to modulate transcription and to induce cell growth arrest, differentiation and apoptosis. HDAC inhibitors also enhance the cytotoxic effects of therapeutic agents used in cancer treatment, including radiation and chemotherapeutic drugs. Marks, P., Rifkind, R. A., Richon, V. M., Breslow, R., Miller, T., Kelly, W. K. Histone deacetylases and cancer: causes and therapies. Nat Rev Cancer, 1, 194-202, (2001); and Marks, P. A., Richon, V. M., Miller, T., Kelly, W. K. Histone deacetylase inhibitors. Adv Cancer Res, 91, 137-168, (2004). Moreover, recent evidence indicates that transcriptional dysregulation may contribute to the molecular pathogenesis of certain neurodegenerative disorders, such as Huntington's disease, spinal muscular atrophy, amyotropic lateral sclerosis, and ischemia. Langley, B., Gensert, J. M., Beal, M. F., Ratan, R. R. Remodeling chromatin and stress resistance in the central nervous system: histone deacetylase inhibitors as novel and broadly effective neuroprotective agents. Curr Drug Targets CNS Neurol Disord, 4, 41-50, (2005). A recent review has summarized the evidence that aberrant histone acetyltransferase (HAT) and histone deacetylases (HDAC) activity may represent a common underlying mechanism contributing to neurodegeneration. Moreover, using a mouse model of depression, Nestler has recently highlighted the therapeutic potential of histone deacetylation inhibitors (HDAC5) in depression. Tsankova, N. M., Berton, O., Renthal, W., Kumar, A., Neve, R. L., Nestler, E. J. Sustained hippocampal chromatin regulation in a mouse model of depression and antidepressant action. Nat Neurosci, 9, 519-525, (2006).
There are 18 known human histone deacetylases, grouped into four classes based on the structure of their accessory domains. Class I includes HDAC1, HDAC2, HDAC3, and HDAC8 and has homology to yeast Rpd3. HDAC4, HDAC5, HDAC7, and HDAC9 belong to class IIa and have homology to yeast Hda1. HDAC6 and HDAC10 contain two catalytic sites and are classified as class IIb. Class III (the sirtuins) includes SIRT1, SIRT2, SIRT3, SIRT4, SIRT5, SIRT6, and SIRT7. HDAC11 is another recently identified member of the HDAC family and has conserved residues in its catalytic center that are shared by both class I and class II deacetylases and is sometimes placed in class IV.
HDACs have been shown to be powerful negative regulators of long-term memory processes. Nonspecific HDAC inhibitors enhance synaptic plasticity as well as long-term memory (Levenson et al., 2004, J. Biol. Chem. 279:40545-40559; Lattal et al., 2007, Behav Neurosci 121:1125-1131; Vecsey et al., 2007, J. Neurosci 27:6128; Bredy, 2008, Learn Mem 15:460-467; Guan et al., 2009, Nature 459:55-60; Malvaez et al., 2010, Biol. Psychiatry 67:36-43; Roozendaal et al., 2010, J. Neurosci. 30:5037-5046). For example, HDAC inhibition can transform a learning event that does not lead to long-term memory into a learning event that does result in significant long-term memory (Stefanko et al., 2009, Proc. Natl. Acad. Sci. USA 106:9447-9452). Furthermore, HDAC inhibition can also generate a form of long-term memory that persists beyond the point at which normal memory fails. HDAC inhibitors have been shown to ameliorate cognitive deficits in genetic models of Alzheimer's disease (Fischer et al., 2007, Nature 447:178-182; Kilgore et al., 2010, Neuropsychopharmacology 35:870-880). These demonstrations suggest that modulating memory via HDAC inhibition have considerable therapeutic potential for many memory and cognitive disorders.
The role of individual HDACs in long-term memory has been explored in two recent studies. Kilgore et al. 2010, Neuropsychopharmacology 35:870-880 revealed that nonspecific HDAC inhibitors, such as sodium butyrate, inhibit class I HDACs (HDAC1, HDAC2, HDAC3, HDAC8) with little effect on the class IIa HDAC family members (HDAC4, HDAC5, HDAC7, HDAC9). This suggests that inhibition of class I HDACs may be critical for the enhancement of cognition observed in many studies. Indeed, forebrain and neuron specific overexpression of HDAC2, but not HDAC1, decreased dendritic spine density, synaptic density, synaptic plasticity and memory formation. (Guan et al., 2009, Nature, 459:55-60). In contrast, HDAC2 knockout mice exhibited increased synaptic density, increased synaptic plasticity and increased dendritic density in neurons. These HDAC2 deficient mice also exhibited enhanced learning and memory in a battery of learning behavioral paradigms. This work demonstrates that HDAC2 is a key regulator of synaptogenesis and synaptic plasticity. Additionally, Guan et al. showed that chronic treatment of mice with SAHA (an HDAC 1, 2, 3, 6, 8 inhibitor) reproduced the effects seen in the HDAC2 deficient mice and rescued the cognitive impairment in the HDAC2 overexpressing mice.
The inhibition of HDAC2 (selectively or in combination with inhibition of other class I HDACs; as the primary target, or as part of a complex with other proteins) is an attractive therapeutic target. Selective inhibition might be achieved by targeting specific HDAC isoforms such as HDAC2, in isolation, or as part of a functional multi-protein complex. Such inhibition has the potential for enhancing cognition and facilitating the learning process through increasing synaptic and dendritic density in neuronal cell populations. In addition, inhibition of specific HDACs, such as HDAC2, may also be therapeutically useful in treating a wide variety of other diseases and disorders.